3GPP that is a standardization group of W-CDMA is studying a communication scheme that becomes a successor to W-CDMA and HSDPA, that is, 3GPP is studying Long Term Evolution (LTE). As radio access schemes, OFDM (Orthogonal Frequency Division Multiplexing) is being studied for downlink, and SC-FDMA (Single-Carrier Frequency Division Multiple Access) is being studied for uplink (refer to non-patent document 1, for example).
OFDM is a scheme for dividing a frequency band into a plurality of narrow frequency bands (subcarriers) so as to perform transmission by carrying data on each subcarrier. By arranging the subcarriers on frequencies densely without interfering with each other while a part of them overlap, high speed transmission is realized so that efficiency of use of frequency increases.
SC-FDMA is a transmission scheme that can decrease interference among terminals by dividing frequency band and performing transmission using frequency bands that are different among a plurality of terminals. Since the SC-FDMA has characteristics that variation of transmission power becomes small, low power consumption in the terminal and wide coverage can be realized.
Generally, in mobile communications, pilot signals are used for channel estimation and radio quality measurement. The pilot signal is called Downlink Reference Signal (DL RS) in LTE.
The downlink reference signal in LTE is represented as a two dimensional sequence, and is formed by a two dimensional orthogonal sequence and a two dimensional pseudo random sequence. Mapping (subcarrier number) of the reference signal to physical resources can be represented by the following equation (non patent document 2):
      k    =                  6        ⁢                                  ⁢        m            +                        (                      v            +                                          f                hop                            ⁡                              (                                  ⌊                                      i                    /                    2                                    ⌋                                )                                              )                ⁢        mod        ⁢                                  ⁢        6                  l    =          {                                    0                                              n              =              0                                                                                          N                symb                DL                            -              3                                                          n              =              1                                          wherein k indicates a subcarrier number, 1 indicates an OFDM symbol number, and i indicates a slot number, and m is an integer value as follows.
            m      =      0        ,    1    ,    …    ⁢                  ,          ⌊                        N          BW          DL                                      N            BW            RB                    /          2                    ⌋            n    =          {                                                                                                        0                    ,                    1                                                                                                                                                                for                          ⁢                                                                                                          ⁢                          p                                                =                        0                                            ,                      1                                        ⁢                                                                                                                                                              0                                                                                                                    for                        ⁢                                                                                                  ⁢                        p                                            =                      2                                        ,                                          3                      ⁢                                                                                          ⁢                      and                      ⁢                                                                                          ⁢                      the                      ⁢                                                                                          ⁢                      generic                      ⁢                                                                                          ⁢                      frame                      ⁢                                                                                          ⁢                      structure                                                                                                                                        0                    ,                    1                                                                                                                                      for                        ⁢                                                                                                  ⁢                        p                                            =                      2                                        ,                                          3                      ⁢                                                                                          ⁢                      and                      ⁢                                                                                          ⁢                      the                      ⁢                                                                                          ⁢                      alternative                      ⁢                                                                                          ⁢                      frame                      ⁢                                                                                          ⁢                      structure                                                                                            ⁢                                                  ⁢                          N              BW              RB                                =          12                ,                              N            symb            DL                    =          7                    NBWDL indicates the number of subcarriers in the whole system band. NBWDL is 300 when the system bandwidth is 5 MHz, is 600 when the system bandwidth is 10 MHz, and is 1200 when the system bandwidth is 20 MHz. P indicates an antenna port number. When only one antenna is used, p=0. When four antennas can be used, p=0, 1, 2 or 3.
In the above-mentioned equation, v is determined by the following equation.
  v  =      {                                        3            ⁢                                                  ⁢            n                                                              for              ⁢                                                          ⁢              p                        =            0                                                            3            +                          3              ⁢                                                          ⁢              n                                                                          for              ⁢                                                          ⁢              p                        =            1                                                            1            +                          3              ⁢                              (                                  i                  ⁢                                                                          ⁢                  mod                  ⁢                                                                          ⁢                  2                                )                                                                                        for              ⁢                                                          ⁢              p                        =            2                                                            2            +                          3              ⁢                              (                                  i                  ⁢                                                                          ⁢                  mod                  ⁢                                                                          ⁢                  2                                )                                                                                        for              ⁢                                                          ⁢              p                        =            3                              In the before-mentioned equation, fhop(j) is a cell-specific integer sequence indicating a hopping pattern which changes for each subframe or each slot of the downlink reference signal. That is, by changing fhop(j) for each cell, it becomes possible to map the downlink reference signal to a subcarrier which is different for each cell.
The value fhop(j) may be a fixed value independent of time. When such a fixed value is set to each cell, the downlink reference signal is mapped while being shifted by the fixed value which is different for each cell.
FIG. 1 shows a mapping example of the reference signal. FIG. 1 shows mapping (left side) to physical resources when the antenna port number is 0 (p=0) and fhop(j) is always 0, and shows mapping (right side) to physical resources when the antenna port number is 0 (p=0) and fhop(j) is always 2.
As shown in the figure, in the former case, the downlink reference signal is mapped to k-th (k=6×j (j is an integer equal to or greater than 0)) subcarrier in the first OFDM symbol (1=0). However, in the latter case, the downlink reference signal is mapped to k-th (k=6×j+2 (j is an integer equal to or greater than 0)) subcarrier in the first OFDM symbol (1=0). Accordingly, the downlink reference signal in LTE is mapped to subcarriers which are different for each cell by properly setting fhop(j).
By the way, it is being studied that the user apparatus performs demodulation of 16 QAM and 64 QAM by using information of a fixed value which is a ratio between transmission power per one subcarrier (transmission power density per unit frequency) of the downlink reference signal and transmission power per one subcarrier (transmission power density per unit frequency) of normal data signal (refer to non-patent document 3, for example). The normal data signal is, as a physical channel, the physical downlink shared channel (PDSCH). It is necessary to perform amplitude estimation for demodulation of 16 QAM and 64 QAM. It can be expected to improve estimation accuracy by performing demodulation by using the knowledge that power density difference between the reference signal and the data signal is a fixed value. In this case, since transmission power per one subcarrier of the downlink reference signal is always constant, transmission power per one subcarrier of the normal data signal is also always constant.
Since the downlink reference signal is not transmitted in all OFDM symbols, there is a time range in which the downlink reference signal is transmitted and a time range in which the downlink reference signal is not transmitted. Therefore, if total transmission power of the base station is set to be constant, transmission power density of the data signal may vary for each OFDM symbol. Then, there is fear that the amplitude estimation accuracy is susceptible to deterioration. Methods are proposed for setting transmission power of normal data signal to be constant in the both time ranges regardless of whether the reference signal is included in the OFDM symbol.
In one of the methods, the normal data signal is prohibited from being mapped to a predetermined subcarrier in a time range in which the downlink reference signal is transmitted. Any data is not mapped to the predetermined subcarrier. By reducing subcarriers where the data signal can be mapped, transmission power density of the data signal can be increased by that. Thus, transmission power density of the data signal can be kept constant regardless of whether the reference signal is transmitted. This technique is described in the non-patent document 3, for example.    [Non-patent document 1] 3GPP TR 25.814 (V7.0.0), “Physical Layer Aspects for Evolved UTRA,” June 2006    [Non-patent document 2] 3GPP TR 36.211 (V0.3.1), “Physical Channels and Modulation,” November 2006    [Non-patent document 3] R1-070088, Power Boosting of Reference Signal in E-UTRA Downlink